Cryogenic refrigeration system

ABSTRACT

A cryogenic refrigeration system wherein in a heat exchanger normally disposed in a container and having a Joule-Thompson orifice provided in one end of the heat exchanger to promote liquefaction of a working fluid by expansion, there is included means to control the flow of fluid through the orifice. The refrigeration system is characterized in that flow through the orifice is regulated by a valve which is activated by a temperature sensing fluid filled bulb-activating means combination spaced apart from the orifice toward the warm end of the heat exchanger.

United States Patent [191 Longsworth [451 Apr. 24, 1973 CRYOGENIC REFRIGERATION SYSTEM [75] Inventor: Ralph C. Longsworth, Allentown,

[73] Assignee: Air Products and Chemicals, Inc.,

Allentown, Pa.

[22] Filed: Dec. 6, 1971 [21] Appl. No.: 204,883

[52] US. Cl. ..62/222, 62/223, 62/514 [51] Int. Cl. ..F25b 41/04 [58] Field of Search ..62/222, 223, 514

[56] References Cited UNITED STATES PATENTS Campbell ..62/5 14 Turton ..62/514 2/1972 Buller ..62/5 14 9/ 1972 Nicholds ..62/218 Primary ExaminerWilliam .l. Wye Att0rneyRonald B.'Sherer et a1.

[57] ABSTRACT A cryogenic refrigeration system wherein in a heat exchanger normally disposed in a container and having a Joule-Thompson orifice provided in one end of the heat exchanger to promote liquefaction of a working fluid by expansion, there is included means to control the flow of fluid through the orifice. The refrigeration system is characterized in that flow through the orifice is regulated by a valve which is activated by a temperature sensing fluid filled bulb-activating means combination spaced apart from the orifice toward the warm end of the heat exchanger.

8 Claims, 5 Drawing Figures PATENTEDAPR 24 I975 sum 1 OF 3 INVENTOR.

h n w. 0 N w m m w A C h w R PATENTEDAPR 24 1973 SHEET 2 OF 3 INVENTOR. Ralph C. Longsworm.

ATTORNEY PATENTEDAPR 24 915 8mm 3 OF 3 h m n 0 T w N E s V g N m w: L Q ww fi wwwmwwm 0 W109 11w m. N9 m w: @IIMRinNS 9 I I w R ATTORNEY CRYOGENIC REFRIGERATION SYSTEM BACKGROUND OF THE INVENTION This invention pertains to cryogenic refrigeration systems, most commonly referred to as cryostats and in particular demand-flow cryostats used in cryo-electronic systems such as for cooling infra-red detectors and the like. Such systems are useful in both fixed ground operation and in airborne detection systems. Refrigeration is produced by expansion of a working fluid, normally in a gaseous state and at a temperature below its inversion temperature, through a Joule- Thompson orifice to produce a supply ofliquefied fluid which is collected in a reservoir and maintained as source refrigerant.

Demand flow cryostats of the type wherein flow control is achieved by sensing the presence or absence of liquid nitrogen at the cold end of the heat exchanger and using the sensing device to control the size of the Joule-Thompson orifice are shown in U. S. Pat. Nos. 3,269,140, 3,413,819 and 3,517,525. In these devices operation is normally in an on-off mode because the sensing mechanism is in contact with the liquefied working fluid so that before the sensor will react it must be warmed above the temperature of the liquid at the top of the insulating dewar within which such cryostats are mounted. U. S. Pat. Nos. 3,320,755 and 3,457,730 show cryostats that employ a sensing device which in turn activates the control mechanism, and extends for almost the entire length of the cryostat. These devices are sensitive to changes in ambient temperature and will therefore operate efficiently only over a narrow range of ambient conditions.

SUMMARY OF THE INVENTION In order to provide an improved demand flow cryostat and avoid the problems of the prior art devices it has been discovered that these goals are realized when a fluid filled sensing element having low thermal mass and high heat transfer rate characteristics is coupled to an activator that controls a valve which in turn regulates fluid flow through the Joule-Thompson orifice and the sensing element is located away from the orifice and between it and the warm end of the cryostat.

It has been found that with the sensing bulb in this location the level of liquid rises to a location near the cold extremity of the sensing bulb and this level is the operative control condition. Variations in level about this point adjust for changes in gas pressure, ambient temperature, heat load, and working fluid.

Therefore it is a primary object of this invention to provide an improved cryogenic refrigeration system.

It is another object of this invention to provide a demand flow cryogenic refrigeration system.

It is still another object of this invention to provide a demand flow cryostat.

It is yet another object of this invention to provide a cryogenic refrigeration system of minimum initial cooldown and fast response characteristics.

It is a further object of this invention to provide a control device that will function over a wide range of ambient temperatures.

It is still a further object of this invention to provide a sensing device that will operate effectively in any orientation.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a plot of temperature against location along the length of a cryostat in a dewar for three different J oule-Thompson orifice openings.

FIG. 2 is a longitudinal section through a cryostat of FIG. 1.

FIG. 3 is a longitudinal section through a cryostat according to a second embodiment of the present inventron.

FIG. 4 is a longitudinal section through a cryostat employing a third embodiment of the present invention.

FIG. 5 is a longitudinal section through a cryostat employing a fourth embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION Referring to the drawing there is shown in F IG. 1 a cryostat 10 consisting of a mandrel 12 around which is wound a finned tube heat exchanger 14. Heat exchanger 14 has on the warm end 16 an inlet conduit 18 which is connected to a source of high pressure working fluid which is normally in the gaseous state. On the cold end 20 there is a Joule-Thompson orifice 22 and a needle valve 24 for controlling flow through orifice 22. The cryostat 10 is normally disposed in a glass dewar 26. Dewar 26 contains an inner flask 28 which receives the cryostat l0 and forms a fluid reservoir 30 between the bottom of the flask 28 and the cold end 20 of the cryostat l0. Curve A of FIG. 1 is a plot of temperature against location along the cryostat when the orifice 22 is sized or throttled so that there is insufficient fluid flowing through the heat exchanger 14 to permit liquefaction of the fluid by expansion. Curve B of FIG. 1 is a similar plot showing the proper orifice 22 opening to permit liquid to form under a minimum flow of working fluid. Curve C of FIG. -1 is another plot of temperature against position showing orifice 22 being open too far with an excess amount of liquid being formed that flows up toward the warm end 16 of the heat exchanger 14 before it is vaporized.

In designing demand flow cryostats it is desirable to maintain a plot of temperature against position such as curve B of FIG. 1 so that sufficient liquid is produced and that a liquid inventory is always present in the bottom of flask 28 under minimum fluid flow through heat exchanger 14. As has been set forth above and will be shown later, in order to achieve this operating condition, it is preferable to have the sensing device that in turn causes an activator to adjust needle valve 24 located within that portion of the length of the cryostat denoted as 0 in FIG. 1.

The cryostat 10 is shown in FIG. 2 enlarged and in section, the dewar being eliminated for clarity. The heat exchanger 14 consists of tube 15 preferably of stainless steel having a plurality of fins 17 mounted transversely of tube 15. The finned portion of tube extends from warm end 16 to the cold end 20 of the cryostat 10 and is disposed around a hollow elongate mandrel 12. On the warm end 16 of the cryostat l0 tube 15 extends for a sufficient length and is provided with a suitable filling (not shown) for connection to a source of working fluid under high pressure. On the cold end 20 the heat exchanger 14 tube 15 is connected in fluid tight relation with an orifice block 21 containing Joule- Thompson orifice 22.

Slidably disposed with mandrel 12 is needle valve block 23 containing needle valve 24, block 23 positioned so that the end 25 of valve 24 is adjacent orifice 22 to control the flow of fluid therethrough. A thin walled tube 32, having a wall thickness of about 0.003 inches, is affixed to the valve block 23 and extends upwardly and has affixed thereto a flexible bellows 34. Between the bellows 34 and valve block 23 the space defined by tube 32 is the temperature sensing bulb 35. On the upper end of bellows 34 is a cap 36 having a charging tube 38 extending in fluid tight relation through the cap 36 into bellows 34. The charging tube 38 extends upwardly through cup 36 into bellows extension tube 40 having a cap 42 which tube 40 is affixed to mandrel 12 as by spot welding at the upper end thereof. The charging tube 38 has a crimped end 44 for sealing fluid inside the bellows 34 and sensing bulb 35.

The sensing bulb 35 and bellows 34 are charged at room temperature with a fluid such as nitrogen gas through fill tube 38 so that the bellows extends a prescribed distance which for a cryostat as shown should be between 0.006 and 0.012 inches. This results in a control distance of 0.003 to 0.006 for the valve to move to achieve full control under operating conditions. When the cryostat is assembled in this manner and the gas in the sensing bulb contracts the needle valve 24 moves toward orifice block 21 and as the end 25 of valve 24 enters the orifice 22 fluid flow through heat exchanger 14 is throttled. The location of the sensing bulb 35 is approximately to 30 percent of the distance from the cold end 20 toward the warm end 16 of cryostat 10. As shown by the curves of FIG. 1, this is the area where the greatest temperature change occurs as a function of orifice size and liquid level. The sensing bulb 35 being of a thin walled tube in close fit inside mandrel 12 assures good thermal contact between the two. In addition, the bulb 35 has low thermal mass and high heat transfer rate from the shell side of the heat exchanger 14 to the bulb gas thus enabling the control mechanism to respond rapidly to a change in the liquid level inside dewar flask 28. As the liquid level decreases the temperature at the bulb level increases expanding the gas inside the bulb 35 elongating bellows 34 thus moving the end 25 needle valve 24 away from orifice 22 permitting increased fluid to flow therethrough and thus increasing the liquid inventory in flask 28 of dewar 26. The fluid inside the bulb 35 generally remains in the gaseous state while the cryostat is operating because it is located in a temperature region that is normally above the liquefication temperature of the fluid. This provides smooth flow control because the large pressure change that accompanies a small temperature change in the two phase region at the cold end 20 of the cryostat is not present. The bulb 35, in one embodiment, is filled with nitrogen and the needle valve 24 is set so that the nitrogen in the bulb has a temperature of about l40 K which at the contained pressure of about 300 psi is above its liquefaction point. Alternatively helium can be used in the bulb 35 A cryostat such as shown in FIG. 2 when using nitrogen in sensing bulb 35 and adjusted for optimum flow with nitrogen as the refrigerant has been found to run satisfactorily with air, argon and methane as the refrigerant. By adjusting the needle setting, i.e., bellows extension, it is possible to employ a flourocarbon refrigerant such as FREON 14.

There is shown in FIG. 3 a second embodiment of the present invention and one that is suitable for very small diameter mandrels. In the embodiment of FIG. 3 the mandrel 12' has disposed around it a finned tube heat exchanger 14' having an inlet tube extension 15' on one end and on the other end an orifice block 21' and Joule-Thompson orifice 22'. Slidably disposed within the mandrel is a thin walled tube 46 and having affixed at the cold end 20' a valve block 23' which carries needle valve 24', the needle valve being similar to that in FIG. 2. Disposed above and spaced apart from valve block 23 thereby defining a sensing bulb 35 is a cap 48. An extended capillary tube 38' extends from chamber 35 through cap 48 to the warm end of the cryostat into an enlarged bellows 50. The bellows 50 is contained in a cover 52 which affixed to mandrel 12 as by welding. Bellows 50 is in fluid tight engagement with the cover 50 to prevent escape of the sensing bulb fluid. The lower portion or collar 54 of bellows 50 is sealed to a cap 56 which in turn is sealed to thin walled tube 46. As with the cryostat of FIG. 2, the sensing bulb 35 is filled with a fluid such as nitrogen to extend bellows 50 predetermined distance. This bellows-sensor assembly will then function identically as does the bellows-sensor assembly of FIG. 2.

In FIG. 4 there is shown another embodiment of the invention wherein the cryostat is disposed in a dewar 60 having therein stepped flask 62. The mandrel 64 is stepped accordingly and the heat exchanger 66 wrapped around the mandrel 64. At the cold end of the heat exchanger 66 is a Joule-Thompson orifice 68. A thin walled tube 70 is disposed within the mandrel and defines the sensing bulb 72. On the lower end of tube 70 is a valve block 74 and needle valve 76 and on the upper end of tube 70 is bellows 78 having a cap 80. The valve block 74 and bellows 78 are fixed to tube 70 in fluid tight arrangement as by soldering and cap 80 is fixed to bellows 78 and mandrel 64 in a similar manner. Cap 80 includes a bellows fill tube 82 which can be sealed after the bellows-bulb mechanism is filled to the desired level with a fluid. At the upper end 84 of mandrel 64 a fluid fitting 86 is soldered to the mandrel. The fitting 86 includes a filter 88 in a central bore 90 which communicates with the tube 92 of heat exchanger 66 for introducing the working fluid into the heat exchanger 66. Threads 94 on fitting 86 receive a suitable fitting from a source of working fluid under pressure.

Another embodiment of the present invention is shown in FIG. 5, wherein the heat exchanger 96 containing a Joule-Thompson orifice 98 is disposed around a mandrel 100. Mandrel 100 has a fluid tight cap 102 at the end containing projecting conduit 104 of heat exchanger 100. Near the other end inside mandrel 100 there is a second closure 106 containing a depending shaft receiving hollow cylindrical member 108. Slidably disposed within member 108 is shaft 110 which is affixed to valve block 112 which in turn carries needle valve 114, the needle serving to regulate flow through orifice 98 as described in regard to the other embodiments of the invention. Shaft 110 carries one end of bellows 116 which is positioned to define a sensing bulb 118 in the space between the bellows 116 and closure 106. The bellows 116 is sealed to mandrel 100 as by brazing, welding or soldering at 120. In this embodiment the bulb is filled to a predetermining bellows extension, however, the bellows is externally rather than internally pressurized with the same operating results as with the other embodiments of the invention.

In all of the above embodiments there are several advantages over devices of the prior art. A first advantage lies in the fact that low thermal mass of the cryostat permits rapid cool down. The cool. down time is further minimized because the Joule-Thompson orifice is essentially full open until liquid is produced and rises to the level of one or two turns of the heat exchanger, to a location near the cold end of the sensing bulb. By locating the sensor as shown, the cryostat is relatively insensitive to ambient temperature; near optimum flows having been observed at ambient temperatures from 95 to 212 F under pressure of from 1,000 to 5,000 psi using nitrogen as the working fluid. The device of the present invention having an overall fast response minimizes flow of fluid and temperature variations at the cold end. Control of the inventive device being based upon liquid level assures the presence of liquid at the cold end (in the dewar) and minimizes inefficiency of operation at other than optimum heat loads. A cryostat according to the present invention has been shown to have near peak efficiency in a dewar with heat loads varying from 200 MW to 1.5 watts. Finally the gas filled sensing bulb is relatively insensitive to cryostat orientation because temperaturn stratification that does exist in the sensing bulb is not subject to significant convective circulation. If, as in the case with prior art devices, the sensing bulb were to have liquid inside and the cryostat operated in the cold end up position this would cause the liquid to drop to the warm end of the bulb and upset the cryostat operation.

Having thus described my invention what is desired to be secured by Letters of Patent of the United States is contained in the following claims:

What is claimed is:

1. In a cryogenic refrigeration system of the type wherein a working fluid is expanded through an orifice in a heat exchanger, disposed about a mandrel having a warm end and a cold end, normally disposed in a container to produce an inventory of liquid at the cold end in the container adjacent the orifice the improvement which comprises:

a valve member disposed adjacent said orifice to control the fluid flow therethrough; means for moving said valve-in relation to said orifice said means including a fixed fluid filled sensing bulb having low thermal mass and high heat transfer capacity and movable actuating means in fluid communication with said bulb responsive to changes in the volume of fluid in said sensing bulb and actuating means, said sensing bulb and actuating means spaced apart from said orifice and located between said orifice and said warm end of said heat exchanger; whereby said bulb and actuating means react to provide smooth controlled fluid flow through said ori- 2. r l refrigeration system according to claim 1 wherein said actuating means is a flexible bellows.

3. A refrigeration system according to claim 1 wherein the sensing bulb is located between 5 and 30 percent of the distance from the cold end toward the warm end of the heat exchanger.

4. A refrigeration system according to claim 1 wherein the actuating means moves the total distance of between 0.003 and 0.006 inches to achieve total flow control.

5. A refrigeration system according to claim 1 wherein said sensing bulb is a tube having a wall thickness of about 0.003 inches.

6. A refrigeration system according to claim 1 wherein the fluid in the sensing bulb actuates means is nitrogen gas and the valve is set so that at operating pressures of about 300 psi the temperature of the nitrogen is about K.

7. A refrigeration system according to claim 1 wherein the actuating means is spaced apart from said sensing means and in fluid communication therewith by a capillary tube.

8. A refrigeration system according to claim 1 wherein said sensing means and said actuating means are inside said mandrel. 

1. In a cryogenic refrigeration system of the type wherein a working fluid is expanded through an orifice in a heat exchanger, disposed about a mandrel having a warm end and a cold end, normally disposed in a container to produce an inventory of liquid at the cold end in the container adjacent the orifice the improvement which comprises: a valve member disposed adjacent said orifice to control the fluid flow therethrough; means for moving said valve in relation to said orifice said means including a fixed fluid filled sensing bulb having low thermal mass and high heat transfer capacity and movable actuating means in fluid communication with said bulb responsive to changes in the volume of fluid in said sensing bulb and actuating means, said sensing bulb and actuating means spaced apart from said orifice and located between said orifice and said warm end of said heat exchanger; whereby said bulb and actuating means react to provide smooth controlled fluid flow through said orifice.
 2. A refrigeration system according to claim 1 wherein said actuating means is a flexible bellows.
 3. A refrigeration system according to claim 1 wherein the sensing bulb is located between 5 and 30 percent of the distance from the cold end toward the warm end of the heat exchanger.
 4. A refrigeration system according to claim 1 wherein the actuating means moves the total distance of between 0.003 and 0.006 inches to achieve total flow control.
 5. A refrigeration system according to claim 1 wherein said sensing bulb is a tube having a wall thickness of about 0.003 inches.
 6. A refrigeration system according to claim 1 wherein the fluid in the sensing bulb actuates means is nitrogen gas and the valve is set so that at operating pressures of about 300 psi the temperature of the nitrogen is about 140* K.
 7. A refrigeration system according to claim 1 wherein the actuating means is spaced apart from said sensing means and in fluid communication therewith by a capillary tube.
 8. A refrigeration system according to claim 1 wherein said sensing means and said actuating means are inside said mandrel. 